With a global spread of the emission control on automobiles, for purification of hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx) in exhaust gases, there have been used three-way catalysts in which microparticles of precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) are supported on supports such as alumina (Al2O3) as a porous carrier, while the supports are further coated on a substrate such as a honeycomb of Cordierite make.
As an active site of catalyst, precious metals have a reduced active site surface area at high temperatures of a level of several hundreds of degrees centigrade, where a sintering tends to occur. To suppress the sintering, there have been taken controlling the distance between precious metal microparticles and controlling the particle size of precious metal microparticles, to be effective with precious metal microparticles to be great in size to some extent and uniformly dispersed to be supported on support surfaces. However, catalysts for automobiles are exposed typically to 800 to 900° C., and in some cases to high temperatures exceeding 1000° C., constituting the difficulty of keeping a catalytic activity at the time of catalyst production in the state of microparticles.
To this point, in Japanese Patent Publication No. 3,466,856, there has been proposed a catalyst in which a catalytic active particle is supported in a state exposed to a support surface, and the catalytic active particle is covered by the support. Further, in Japanese Patent Unexamined Publication No. 2003-80077, there has been proposed a catalyst in which at least part of a surface of a catalytic base particle of a nanometer order is covered by a precious metal or precious metal oxide, and the surface of the catalytic base particle further has a sintering inhibitor residing thereon.